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The foam solution to ceiling moisture problems

Foaming a vaulted ceiling

Pros spray foaming agents into open spaces. The materials expand and harden in place, forming the insulation layer.

Photo courtesy of Lapolla

Inexplicably, some well-built vaulted ceilings, complete
with vents and fiberglass insulation, have moisture problems. The drywall becomes stained, the insulation becomes damp and the roof wood can even begin to rot.

Foam insulation is an excellent, although expensive
solution. But it’s not a DIY project. There are two
types available: “open” and “closed” cell. The terms refer
to whether the foam bubbles burst during curing, making
the foam soft like a sponge, or remain intact and firm, like
those in the rigid foam panels at home centers.

Closed-cell foam in a vaulted ceiling offers substantial
advantages over open-cell. First, closed-cell foam has a
60 percent higher R-value per inch than open-cell (6.3 vs.
3.9). The higher R-value reduces condensation when
moist interior air hits the now “less-cold” ceiling (vice
versa in summer with humid outside air and A/C inside).
Second, since the closed cells in the foam prevent air
movement and moisture absorption, the foam acts as both
an air barrier and a vapor diffusion retarder (the new
name for a vapor barrier). Open-cell foam restricts air
movement, but it doesn’t prevent vapor diffusion. You
have to install a separate vapor diffusion retarder. Even
then, if moisture gets past it, the open-cell material will
hold it—not a good thing. Finally, closed-cell foam is
more rigid, so it actually strengthens the roof deck. That
added strength restricts flexing from snow loads—the
kind of flexing that creates leak points. Closed-cell foam
costs 40 percent more than open-cell.

But know this. Once water gets into a foam-insulated
space, no matter what type of foam, it doesn’t leave, and
rot happens quickly. So your shingled roof has to be in
good shape and well maintained.